This invention relates to a process and apparatus used in cellulose extrusion and the obtained product, i.e. extruded cellulose film in, for example, sheet or tubular form.
The process relates to extruding a solution of cellulose, water and an amine oxide according to the well-known amine oxide extrusion process. In particular, the process relates to the reduction or avoidance of degradation or discoloration of the extruded cellulose solution, which otherwise leads to poor product quality.
The apparatus also relates to the avoidance of variations in thickness and edge wrinkling of the extruded film.
The apparatus is also suitable for use in extrusion of a blown film into a precipitation medium.
The invention also relates to an extruded cellulose product having a novel structure.
The production of extruded cellulose articles, such as fibers sheets or tubes has been known for more than a century. In this so-called “viscose” process, cellulose is derivatised with carbon disulphide and solubilized in diluted sodium hydroxide to form a solution and the solution is extruded. The extruded cellulose is then regenerated and reverts to its solid form. The viscose process has been used for the manufacture of sausage casings, flat cellulophane films, rayon fibers bottle caps etc. A disadvantage of the viscose process is that it employs carbon disulphide as an intermediate, which is environmentally undesirable.
More recently, the so-called “amine oxide” process has been developed wherein cellulose is dissolved in a mixture of water and an amine-oxide solvent. A commonly used amine-oxide solvent is the tertiary amine-oxide NMMO (N-methyl morpholine N-oxide). This solvent is able to dissolve cellulose without having to first derivatise the cellulose, as for example in the viscose process. Once solubilised the cellulose will precipitate from the solution as a regenerated cellulose product by contacting the solution with a precipitation liquid which is a non-solvent for cellulose and a solvent for NMMO. The most frequently used precipitation liquid for the amine-oxide process is water. Prior to extrusion, the cellulose amine-oxide solution may be heated to a temperature of around 100° C. Thermal stabilisers, such as propylgalate, may be added to the solution to inhibit the thermal degradation of NMMO.
It has now been surprisingly found that using the amine-oxide process, extruded cellulose films having novel structure and enhanced mechanical properties may be obtained.
Furthermore, it has been found when extruding cellulose films, that the extruded material was coloured brown-red. Moreover, the colour intensity varied between different extrusion runs in an apparently uncontrolled manner. Colour variation could even be observed during a single long extrusion run.
In the extrusion step of the amine-oxide process when used to produce tubular casings (for use, for example in the food industry), the tube is extruded into a bath of precipitation liquid. The precipitation liquid is also maintained within the extruded tube so as to solidify the cellulose from the inside. A slight positive pressure of air may be maintained above the precipitation liquid within the extruded tube so as to expand the tube in the transverse direction (ie. transverse to the machine extrusion direction). The presence of precipitation liquid within the extruded cellulose tube causes process difficulties after the tube emerges from the precipitation bath, since it is necessary to cut the tube at regular intervals in order to allow the liquid to drain away. Failure to do so, leads to undesirable stretching of the tube and variations in diameter and thickness.